Recent improvements in durability and reliability of electronic devices for the measurement of temperature, pressure and other parameters have led to widespread use of same in hostile environments such as are encountered by the petroleum industry in well bores. Some of these devices are run into the well bore at the end of an electric cable (commonly referred to as a "wireline") which can provide a power source as well as a real-time retrieval means for data from the device. However, in other instances use of a wireline is precluded due to the necessity of providing a relatively unobstructed flow path for formation fluid, which path would be blocked by an instrument pod in the bore of the pipe string through which the test is to be run. It is also somewhat dangerous to run an instrument pod on a wireline below a shutoff valve in the pipe string, such as a ball valve, because the wireline, if not severed when the ball is rotated to close it during an emergency, may cause a blowout of well fluid by jamming the valve open.
Accordingly, it has become common practice to run measuring devices of the aforesaid type in the well bore in an instrument housing incorporated in the wall of the pipe string either above or below the valve or valves therein, and to record data taken during the testing of the well for subsequent review. Of course, these devices require a power source providing a relatively high power output in a compact size. By way of example, and not limitation, one such battery is the RMM 150 Li/Cl.sub.2 in SO.sub.2 Cl.sub.2 battery, produced by Electrochem Industries of Clarence, N.Y. This battery, as well as other lithium-based batteries, comprise sealed cells which may literally explode when elevated to a certain level above ambient temperature. For example, the aforesaid RMM 150 battery may explode at approximately 420.degree.-425.degree. F. with sufficient force to rupture a housing having a burst pressure in excess of 25,000 psi. Other lithium cells, such as LiSO.sub.2 cells, will explode at even lower temperatures, at about 350.degree. F. In the event of a short circuit or other heating of the battery beyond its limits as noted above, the cell literally becomes a lethal bomb or grenade.
Prior art attempts have been made to design cells that will vent a substantial increase in internal pressure in a safe manner. Such a design is disclosed in U.S. Pat. No. 4,338,382, which design includes a burst diaphragm topped by a ball and split ring or other assembly designed to interrupt circuit continuity in the battery while either preventing escape of the gas inside the cell or venting the gas in a controlled manner. This design, however, assumes that internal pressure will be generated by overcharge or extended discharge of the cell, and does not provide for the situation where a cell may be externally heated, such as may occur in a fire, or in a deep well bore. In such an instance, the "controlled" venting will not prevent explosion of the cell. Another design attempting to address the explosion problem is disclosed in U.S. Pat. No. 4,397,919. This design includes a non-cylindrical cell cross-section to permit bulging under an increase in internal pressure and a venting mechanism which relies upon the melting of a plug at the end of the cell to permit escape of internal pressure above the melting point of the plug. Again, this design primarily addresses the explosion problem due to short-circuiting and overcharge condition, and assumes that the initial bulging of the case and venting of the internal pressure will preclude explosion. Both of these patents, however, fail to address the damage which may be caused by the pressurized venting fluid to objects or personnel in its path, nor do they adequately address a relatively sudden heating of the cell which would build internal pressure much faster than it could be vented.